I. Field of the Invention
The present invention relates to communication. More particularly, the present invention relates to a novel and improved method and apparatus for commanding a modem comprising a wireless link.
II. Description of the Related Art
A variety of apparatus exists today that use standard telephone networks to convey data from one terminal to another. One of the most common terminals that uses a standard telephone network to convey data is the facsimile (FAX) machine. Like other terminal equipment that use standard telephone networks, a FAX machine uses a modem to translate digital information to audible analog tones for transmission over the telephone network. At the receiving FAX machine, a modem is used to convert the tones back into an estimate of the original digital information sent by the transmitting FAX machine.
With the deployment of cellular and personal communications wireless systems, the end user may wish to connect his terminal equipment to a mobile communications device instead of directly to a land telephone network. However, if the user attempts to send the audible tones produced by a modem over a standard analog FM cellular channel, performance will be degraded. In general, the FM cellular channel introduces a substantial amount of noise into the link compared to the standard land telephone network. The additional noise translates into errors in the estimation of the digital information when the signal is converted at the receiving end. The errors on the link can be overcome by constraining communications to lower transmission rates.
If the end user attempts to send the audible tones produced by a modem over a standard digital wireless channel, it is also doubtful that he will be successful. Digital wireless communication equipment typically uses vocoders to convert incoming voice signals into digital bits for transmission over the channel. Vocoders are tailored to sample and compress human voice. Because modem tones differ significantly from human voice, the vocoder can cause noticeable degradation to the modem tones. Also, maximizing the capacity of the system in terms of the number of simultaneous users that can be handled is extremely important in a system using a wireless link. Digitizing modem tones with a vocoder and sending them over the digital wireless link is an inefficient use of the radio channel.
A more efficient, flexible, and reliable transmission means is to provide a mechanism to transmit the digital data over the digital wireless link directly. This configuration presents some unique opportunities to take advantage of the digital link to provide high quality service.
A typical digital wireless link is not an error free channel. To provide error free communication, the wireless link protocol may provide re-transmission of lost or corrupted data. The re-transmissions introduce arbitrary and significant delays in the data. When time sensitive messages are transmitted over the wireless link, the timing of the signals may be lost thus prohibiting proper communications.
Several methods exist today that use standard telephone networks to convey data from one terminal to another. Terminals that connect by telephone networks can be FAX machines, personal computers, credit card verification machines, and telemetry devices. The standard connection of these type of devices is shown in FIG. 1. For example assume terminal equipment 10 is transmitting information to terminal equipment 50. Terminal equipment 10 produces digital data 110 representative of the information. Modem 20 converts digital data 110 to analog signal 120. Analog signal 120 is of a proper bandwidth and power level to be transferable over standard telephone network 30. Telephone network 30 conveys analog signal 120 to its destination. The telephone network may introduce noise such that output audible signal 130 is an estimate of analog signal 120. Modem 40 converts audible signal 130 to digital data 140 which is an estimate of digital data 110. Terminal equipment 50 receives digital data 140 and can act on an estimate of the information sent by terminal equipment 10. Most such links are bi-directional with the reverse functions working in the same manner as the forward.
FIG. 1 is a simplified diagram. Actual implementations of these connections can take on a variety of forms. For example in some equipment, such as a standard FAX machines, the terminal equipment and the modem reside in the same housing. Also telephone network 30 may use any of a variety of methods well known in the art to convey analog signal 120 to its destination. Such methods may include digitizing the signal and transmitting the signal via satellite to a remote location where audible signal 130 is created.
A wireless link is introduced into the configuration of FIG. 1 when a user would like to connect his terminal equipment but does not have access to a land telephone network. The user may instead have a digital mobile communications device. FIG. 2 is an exemplary embodiment of such a configuration. In FIG. 2, modem 20 has been replaced with mobile unit 60, wireless link 160, and digital signal processor (DSP) and modem 70. DSP and modem 70 resides in base station 80 which could be a standard cellular or personal communications base station also capable of handling digital and analog voice communications. FIG. 2 assumes that just one of the connections has been replaced with a wireless link. The description that follows applies equally if communication is achieved with two wireless links.
Referring to FIG. 2, assume again that terminal equipment 10 is transmitting information to terminal equipment 50. Terminal equipment 10 produces digital data 110 representative of the information. Mobile unit 60 modulates digital data 110 and provides output over wireless link 160. The signal from wireless link 160 is received by base station 80 and DSP and modem 70. DSP and modem 70 converts the digital signal to analog signal 120 which is the same signal output by modem 20 in FIG. 1. Telephone network 30 conveys analog signal 120 to its destination. Telephone network 30 may introduce noise such that output audible signal 130 is an estimate of analog signal 120. Modem 40 converts audible signal 130 to digital data 140 which is an estimate of digital data 110. Terminal equipment 50 receives digital data 140 and can act on an estimate of the information sent by terminal equipment 10. This link is bi-directional with the reverse functions working in the same manner as the forward.
Because in FIG. 2 modem 20 has been replaced with mobile unit 60, wireless link 160, and DSP and modem 70, a new partitioning of the functions of modem 20 is necessary. In addition, some new commands and new functionality are necessary to use the wireless link. Ideally this new functionality should operate independent of the terminal equipment so that no modification of the terminal equipment is necessary to support the wireless link. FIG. 3 provides a more detailed representation of an exemplary configuration of mobile unit 60. In FIG. 3, mobile unit 60 is assumed to be a mobile telephone that provides both voice and data capabilities. In an alternative embodiment, mobile unit 60 might be a dedicated single use mobile unit.
In FIG. 3, digital data 110 is connected to protocol stack 230 within mobile unit 60. Protocol stack 230 is capable of bi-directional communication with terminal equipment 10, mobile unit control processor 240 and radio modulator/demodulator 220. When protocol stack 230 receives digital data 110 for transmission over wireless link 160, it provides any required encoding and passes the encoded information to radio modulator/demodulator 220 through switch 210. Radio modulator/demodulator 220 modulates the encoded information and provides the signal for transmission to wireless link 160. Inversely when a signal containing information for terminal equipment 10 arrives, radio modulator/demodulator 220 demodulates the signal and provides it to protocol stack 230 through switch 210.
Likewise when vocoder 200 receives analog information from speaker/microphone 260 for transmission over wireless link 160, it encodes the information and passes the encoded information to radio modulator/demodulator 220 through switch 210. Radio modulator/demodulator 220 modulates the encoded information and provides the signal for transmission to wireless link 160. When a signal containing information for output on speaker/microphone 260 arrives on wireless link 160, modulator/demodulator 220 demodulates the signal and provides it to vocoder 200 through switch 210. Vocoder 200 decodes the signal and provides audible output to speaker/microphone 260.
Mobile unit control processor 240 provides control over the functions of mobile unit 60. In the prior art configuration, information intended for mobile unit control processor 240 might come either over wireless link 160 or from digital data 110. Protocol stack 230 directs information intended for mobile unit control processor 240 to it and receives commands and information for transmission from mobile unit control processor 240. Mobile unit control processor 240 also provides control functions for vocoder 200, switch 210 and radio modulator/demodulator 220.
Protocol stack 230 provides the main control center for data communication with terminal equipment 10 over wireless link 160. Protocol stack 230 may need to provide flow control. For example, digital data 110 may have a data rate that is higher than that of which wireless link 160 is capable. In this case protocol stack 230 may store the excess data and recall data at a rate appropriate for output on the link.
Protocol stack 230 also packetizes and unpacketizes data. Typically digital data 110 provided by terminal equipment 10 is a steady stream of characters. Typically wireless link 160 is packetized meaning that certain bits of information are grouped together for processing and transmission. A group of data is called a frame. Frames aid in error detection and correction. Protocol stack 230 must packetize digital data 110 provided by terminal equipment 10 into frames for radio modulator/demodulator 220 and unpacketize frames from radio modulator/demodulator 220 for digital data 110.
Protocol stack 230 must also recognize an array of special commands. For example, one function of protocol stack 230 is to compensate for the time varying delay of the digital wireless link. Protocol stack 230 recognizes time sensitive messages and translates them into, time insensitive messages for transmission over the wireless link. At the base station, the time insensitive messages are recognized and the time sensitive messages may be reconstructed with the appropriate timing.
Referring again to FIG. 1, terminal equipment 10 can produce several categories of output. Terminal equipment 10 can produce information for transfer to terminal equipment 50. Terminal equipment 10 can produce commands for modem 20 which are not to be transferred to terminal equipment 50. Because in FIG. 2, modem 20 of FIG. 1 has been replaced with mobile unit 60, wireless link 160, and DSP and modem 70, if terminal equipment 10 sends a message intended for the portion of the modem contained in the base station, DSP and modem 70 must act on command. Conversely, if terminal equipment 10 sends a message intended to control the portion of the modem contained in the mobile unit or to control the telephone functions of mobile unit 60, the message must be acted upon by mobile unit 60.
In FIG. 2, when a link is established between terminal equipment 10 and terminal equipment 50, the system is said to be in data active mode. When such a link does not exist, the system is said to be in data idle mode. In data mode and data active mode, DSP and modem 70, provide analog signal 120 in response to digital data 110.
In FIG. 2, when terminal equipment 10 sends a message intended solely for mobile unit 60 and/or DSP and modem 70, the system is said to be in command mode. In this mode, terminal equipment 10 may send command instructions or set parameters directly with mobile unit 60 and DSP and modem 70. The commands messages are not transferred into analog signal 120. Command mode can be invoked independent of an established data connection. Therefore three possible data states exist: command mode and data idle, command mode and data active, data mode and data active.
Typical prior art systems invoke command mode by a recognizable escape sequence such as described in U.S. Pat. No. 4,549,302, issued Oct. 22, 1985, entitled “MODEM WITH IMPROVED ESCAPE SEQUENCE MECHANISM TO PREVENT ESCAPE IN RESPONSE TO RANDOM OCCURRENCE OF ESCAPE CHARACTER IN TRANSMITTED DATA”, assigned to Hayes Microcomputer Products, Inc. Referring again to FIG. 1, terminal equipment 10 sends the unique escape sequence indicating that modem 20 should enter command mode. Modem 20 recognizes the escape sequence and analyses any subsequent messages as commands.
In the system of FIG. 2, the escape sequence is issued in an identical manner from terminal equipment 10 which lends itself to an undesirable situation. In the prior art configuration, both mobile unit 60 and DSP and modem 70 must recognize the escape to command mode sequence. Both mobile unit 60 and DSP and modem 70 must be in the same mode to insure proper operation. For example, during an active data mode operation, terminal equipment 10 may need to command the telephone functions of mobile unit 60 for example changing operating parameters such as flow control and answer mode. To do so, terminal equipment 10 sends a message to switch mode from data mode to command mode. If either DSP and modem 70 or mobile unit 60 does not respond to the change in command, an unrecoverable error can occur. For example, if mobile unit 10 remains in data mode, it will continue to pass the commands intended for it over wireless link 160. DSP and modem 70, even though in command mode, is unable to act upon the command intended for mobile unit 10.
Another adverse result of the two state system is that a base station may be equipped to respond to commands unknown to the mobile unit. The base station also may be updated periodically with new command sets such as specialty commands associated with a single carrier. The mobile units may not be updated to recognizes these commands. If terminal equipment 10 attempts to send a new command to DSP and modem 70 which is unrecognized by mobile unit 60, mobile unit 60 may respond with an “error” message back to terminal equipment 10 instead of passing the unrecognized command over wireless link 160. Alternatively mobile unit 60 may pass the signal unrecognized to DSP and modem 70. In this case if a new command unknown to mobile unit 60 is created that indicates that the state of the system should change from command mode to data mode, mobile unit 60 remains in command mode and passes the command to DSP and modem 70. DSP and modem 70 responds by entering data mode thus creating a system having a mismatched state.
The complexity of the system is increased by the need to have both mobile unit 10 and DSP and modem 70 have a matching state. The probability of an error occurring when transistioning from one state to another also increases. Therefore to decrease the complexity and increase the reliability of the system, it is advantageous to eliminate the dual state requirement of the prior art configuration.
It is therefore the object of the present invention to provide an efficient method and apparatus for passing commands in a system comprising a wireless link.
It is a further object of the present invention to remove the dual state requirement for proper operation.